This invention relates to a ghost elimination device for removing, from a television signal, a ghost component created by the distortion of the transmission path of the television signal.
In receiving a television signal broadcasted by a television station, the distortion of transmission path caused by reflection waves, which are created by such obstacles as tall buildings and mountains, superimposed on the direct wave is called "ghost", and it is a major cause of deteriorated picture quality in the ground television broadcasting system.
In order to regain the picture quality which has been spoiled by the ghost, there have been developed methods and devices for eliminating the ghost, in which a reference signal for ghost elimination is transmitted from the broadcasting station and the signal is used by receivers to detect and remove the ghost, as described in the Technical Report of the Institute of Television Engineers of Japan, Vol. 13, No. 32, pp. 1-36, published in June 1986.
In general, a television signal received by a receiver is presumed to include a great deal of noise, and accordingly ghost information detected based on the reference signal very likely includes errors, and therefore the performance of ghost elimination will be degraded.
A means of overcoming this problem is known, as described in Japanese Patent Publication No. 62-22307. This technique is based on the provision of a noise elimination circuit connected to the output of a distortion elimination filter which suppresses the transmission path distortion (ghost) and a control circuit which receives the output of the noise elimination circuit to control the filter thereby to suppress the noise included in the reference signal for the achievement of error prevention in the ghost information.
With regard to the reference signal for ghost elimination, the above-mentioned technical report describes, in its paragraph on page 31 for the principle of ghost elimination, that ghost up to about 4 .mu.m can be detected through the computational process for the GCR (Ghost Cancel Reference) signal.
The ghost elimination device using the GCR signal employs a control circuit connected to the output of a distortion elimination filter (transversal filter) which suppresses the transmission path distortion (ghost), and the control circuit processes the GCR signal to detect the distortion information of the transmission path and manipulates the tap factor of the distortion elimination filter thereby to remove the ghost.
However, the foregoing prior art involves a problem of increased time needed for the removal of distortion (removal of ghost). The above-mentioned noise elimination circuit bases its noise suppression on synchronous addition which utilizes the randomness of noise generation among fields in contrast to the significant signal that has a correlation among fields, and its n-time synchronous addition fields an s/n improvement factor of .sqroot.n, i.e., an s/n improvement of about 20 dB requires synchronous addition 100 times.
The above-mentioned control circuit calculates the tap factor of the transversal filter, which constitutes the distortion elimination filter, from the reference signal with its noise being suppressed, and revises the tap factor accordingly. This operation takes place many times iteratively, and in consequence the distortion of the transmission path is eliminated.
The filter has different characteristics before and after the revision of tap factor and the correlation is lost for distortions impressed on the reference signal, and therefore the noise elimination circuit cannot use the results of synchronous addition obtained up to the previous factor revision. This situation requires n-time synchronous addition at each revision of tap factor, resulting in an extended noise elimination time.
The GCR signal is formed in a sequence of patterns which cycles in eight fields, as shown in FIG. 1 on page 31 of the above-mentioned Technical Report of the Institute of Television Engineers of Japan, Vol. 13, No. 32, and also will be explained later in this specification, in order to avoid erroneous detection due to the mixing of a distortion component of the previous line. A signal inserted to the previous line is the VIT (Vertical Interval Test) signal, which has a fixed pattern at least among even fields and among odd fields each.
For the detection of the transmission path distortion from the GCR signal by being free from the influence of the distortion of the previous line, it is necessary to decode the transmission sequence of the GCR signal by using signals of eight fields through such computation as (S1-S5)+(S6-S2)+(S3-S7)+(S8-S4), as will be explained in detail later.
For the suppression of distortion on the transmission path, the above-mentioned control circuit calculates the tap factor of the distortion elimination filter based on the signal produced by the above computation process, and updates the tap factor. This operation takes place many times iteratively, and in consequence the distortion of the transmission path is eliminated.
The filter has different characteristics before and after the revision of tap coefficient factor and the correlation of distortions impressed on the GCR signal is lost. On this account, the use of signals before and after the tap factor revision for the above computation process will result in a faulty detection of the distortion information. Otherwise, it takes a wait time of eight fields for the decoding operation at each revision of tap factor, resulting in a long distortion elimination time.